It has conventionally been known that a polystyrene-based resin expanded molded article obtained by forming polystyrene-based resin pre-expanded particles in a mold is excellent in rigidity, heat insulating property, lightweight property, water-resistant property, and expansion moldability. However, the expanded molded article has a drawback of being poor in chemical resistance and impact resistance. The expanded molded article also has a drawback of generating much odor, which makes it difficult to apply the expanded molded article to a usage of an interior of an automobile.
On the other hand, it has been known that an expanded molded article made of the polyolefin-based resin has reduced odor, and is excellent in chemical resistance and impact resistance. Therefore, the expanded molded article is used for automobile-related components. However, since the polyolefin-based resin is poor in a retention capacity of a blowing gas, an expansion molding condition has to be precisely controlled, which entails a problem of an increased production cost. Additionally, the expanded molded article made of the polyolefin-based resin also has a drawback of being poor in rigidity, compared to a styrene-based resin expanded molded article.
In view of the circumstance described above, an expanded molded article having excellent chemical resistance and rigidity has been produced from composite resin particles containing the polystyrene-based resin and the polyolefin-based resin, so that wherein the drawbacks of both the polystyrene-based resin expanded molded article and the polyolefin-based resin expanded molded article are overcome. However, such expanded molded article described above is still required to have a lower level of odor content, when used for automobile-related components (in particular, an interior material of an automobile).
It has been found that the odor is generated from a raw material of the composite resin particles or generated according to a production method thereof. Therefore, there is a method of reducing odor through a treatment after the production of the composite resin particles, in order to reduce the odor. For example, Japanese Unexamined Patent Application No. HEI 10 (1998)-195129 (Patent Document 1) discloses a method of reducing a volatile component such as an unreacted monomer from polystyrene-based polymer particles.
This document describes that a drying process is performed at a drying temperature of 200° C. or less until the ratio of the volatile component to be reduced falls to 20 wt. % or less, but this document does not relate to a method of reducing odor.